A lithographic apparatus is a machine that applies a desired pattern onto a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto an exposure area (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent exposure areas that are successively exposed. Known lithographic apparatus include so-called steppers, in which each exposure area is irradiated by exposing an entire pattern onto the exposure area in one go, and so-called scanners, in which each exposure area is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
There is an ongoing desire in the lithography industry to provide improved control of the polarization and/or the pupil plane distribution of radiation projected onto a substrate (these properties are sometimes referred to in combination as being the illumination mode of the radiation). It is desirable to provide a lithographic apparatus and method which allows this to be achieved.